Proportioning valve with pressure limiting

ABSTRACT

A proportioning valve is installed in the rear brake line of the vehicle equipped with separate brake hydraulic systems for the front and rear brakes. The proportioning valve permits uninhibited fluid communication to the rear wheel brakes until a first pressure level is attained whereupon the valve restricts flow of fluid to the rear wheel brakes to establish a lower fluid pressure level in the latter than the fluid pressure level communicated to the front wheel brakes. An additional valve in the rear brake line is responsive to the pressure differential across the proportioning valve to terminate fluid communication to the rear wheel brakes when the pressure differential across the proportioning valve attains a predetermined amount. This feature enables the relationship between the braking pressure delivered to the rear wheel brakes and the braking pressure delivered to the front wheel brakes to approximate closely the ideal pressure relationship. In one embodiment of the invention, the point at which fluid communication to the rear wheel brakes is initially restricted by the proportioning valve varies as the rate of deceleration of the vehicle varies. In a second embodiment of the invention, the pressure level at which the proportioning valve initially restricts fluid pressure communication to the rear wheel brakes varies as the load carried by the rear axle of the vehicle varies.

United States Patent 1 1 Lewis 1 1111 3,773,362 Nov. 20, 1973PROPORTIONING VALVE WITH PRESSURE LIMITING [75] Inventor: Richard L.Lewis, South Bend, Ind.

[73] Assignee: The Bendix Corporation, South Bend, Ind.

22 Filed: Dec. 1,1971 21 Appl. No.: 203,712

52 U.S. Cl. 303/6 (1, 188/349 51 Int. Cl... B60t 13/00 [58] Field ofSearch 303/6 C; 188/349 [56] References Cited UNITED STATES PATENTS3,671,080 6/1972 Kawaguchi 303/6 c 3,680,922 8/1972 Kawal 303/6 cPrimary Examiner-Allen N. Knowles Attorney-Ken C. Decker et al.

[57] ABSTRACT A proportioning valve is installed in the rear brake lineof the vehicle equipped with separate brake hydraulic systems for thefront and rear brakes. The proportioning valve permits uninhibited fluidcommunication to the rear wheel brakes until a first pressure level isattained whereupon the valve restricts flow of fluid to the rear wheelbrakes to establish a lower fluid pressure level in the latter than thefluid pressure level communicated to the front wheel brakes. Anadditional valve in the rear brake line is responsive to the pressuredifferential across the proportioning valve to terminate fluidcommunication to the rear wheel brakes when the pressure differentialacross the proportioning valve attains a predetermined amount. Thisfeature enables the relationship between the brakingpressure deliveredto the rear wheel brakes and the braking pressure delivered to the frontwheel brakes to approximate closely the ideal pressure relationship. Inone embodiment of the invention, the point at which fluid communicationto the rear wheel brakes is initially restricted by the proportioningvalve varies as the rate of deceleration of the vehicle varies. In asecond embodiment of the invention, the pressurelevel at which theproportioning valve initially restricts fluid pressure communication tothe rear wheel brakes varies as the load carried by the rear axle of thevehicle varies.

8 Claims, 3 Drawing Figures PATFINIEUHUVQO ma 3.773.362

- sum 2 U? 2 F c /52 4 5555s I I I32 I5 A a AZ /53 J2 130 1 9 gm Q 124,lY/Xi I12 135 I24 I4 I22 I 114 HEAVY VEHICLE WEIGHT D B REAR SHUT OFF9', ASLIGHT VEHICLE WEIGHT FRONT Po FIGS INVENTOR. RICHARD L. LEWISATTORNEY This invention relates to a pressure control valve device foruse in a vehicle braking system.

The ideal relationship between the brake pressure 1 delivered to thefront wheel brakes of the vehicle to the employ pressure controldevices, such as proportioning valves in the rear brake lines of avehicle. Since the braking pressure required by the rear wheel brakes inproportion to the braking pressure required by the front wheel brakesvaries as the load carried by the rear axle of the vehicle varies, ithas become customary to employ a proportioning valve that is responsiveto the load carried by the rear axle of the vehicle, particularly incommercial vehicles. As is evident to thoseskilled in the art, it isalso possible to obtain the same effect as a load sensing proportioningvalve by providing a valve which is responsive to the rate ofdeceleration of the vehicle, since the fluid pressure required to obtaina predetermined deceleration rate will vary as the load carried by thevehicle varies. However even these load sensing proportioning valveshave not closely approximated the ideal pressure relationship betweenthe front and rear brakes, because after the pressure level delivered tothe front and rear brakes reaches a certain value, it is desirable notto increase further the pressure level delivered to the rear wheelbrakes as the pressure level to the front wheel brakes is increased.

SUMMARY OF THE INVENTION Therefore, an important object of my inventionis to provide a pressure control device for use in a vehicle hydraulicbraking system which establishes a pressure relationship between thefront and rear wheel brakes which closely approximates the idealpressure relationship between the latter. 7

Another important object of my invention is to provide a proportioningvalve for use in the rear wheel brake lines which permits uninhibitedfluid communication to the rear wheel brakes until a predeterminedpressure level is attained, which then restricts fluid communication toestablish a lower pressure level at the outlet of the valve which iscommunicated to the rear wheel brakes to the pressure level establishedat the inlet of the valve, and which terminates fluid communication tothe rear wheel brakes when the pressure differential established acrossthe valve attains a predetermined value.

Still another important object of my invention is to provide a valvewhich is responsive to the load carried by the vehicle and the rate ofdeceleration of the latter to vary the pressure level at which fluidcommunication is initially restricted to the rear wheel brakesaccordingly, while maintaining substantially constant the predeterminedpressure differential across the valve at which fluid communication tothe rear wheel brakes is terminated.

DESCRIPTION OF THE DRAWINGS the teachings of my present inventionillustrated in cross section;

FIG. 2 is a cross-sectional view of an alternate embodiment of thecontrol valves illustrated in FIG. 1; and FIG. 3 is a diagrammaticillustration of the operation of the valves illustrated in FIGS. 1 and2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to theFIG. 1 of the drawings, a control valve generally indicated by numeral10 includes a housing 12 defining a bore 14 therewithin having an inletport 16. The inlet port 16 is communicated to one side of a standardautomotive split system master cylinder generally indicated by thenumeral 18, the other side of which is communicated to the front wheeldisc brakes 20 of the vehicle. Fluid pressure is developed in the mastercylinder 18 in the conventional manner by operation of a brake pedal 22mounted in the vehicle operators compartment.

First and second valve members 24 and 26 are slidably mounted in thebore 14 and divide the latter into an inlet chamber 28 and an outletchamber 30. Valve member 24 is stepped to define a larger effective area32 exposed to the fluid pressure level in the outlet chamber 30 and asmall-er effective area 34 exposed to the fluid pressure level in theinlet chamber 28. Arm members 36 extend from end 34 of first valvemember 24 and are provided with suitable detents that are adapted toengage corresponding detents on arms 38 which are carried by the secondvalve member 26. Engagement of the detents carried by the arm members 36and 38 limits relative movement between the valve members 24 and 26 to apredetermined distance. A spring 40 is disposed between the members 44and 26 and yieldably urges the detents on the arm members 36 intoengagement with the corresponding detents on the arm members 38. Anotherspring 42 yieldably urges the end 32 of the valve member 24 intoengagement with the right hand end of the housing 12, viewing FIG. 1.

Valve member 26 includes a housing 44 which is slidably received withina guide member 46 mounted in the bore 14. A relatively dense mass 48 isslidably received within the housing 44 and is urged toward the righthand end of housing 44 viewing FIG. 1 of the latter by a spring 50. Aresilient washer 52 includes projecting arms which extend throughopenings 54 provided in the housing 44 and which terminate in taperedends 56 which are adapted to be urged into locking engagement with oneof serrations 58 provided on the guide member 46 upon movement of themass to the left, viewing FIG. 1, against the force of the spring 50during a vehicle deceleration. Second valve member 26 further includes aresilient poppet member 61 which is movable relative to the mass 48 butwhich is yieldably urged into engagement with a stop member 60 forming aportion of the housing 44 by a spring 62. The poppet member 61 isadapted to sealingly engage passage 64 extending through the first valvemember 24 to control fluid communication through the latter between theinlet chamber 28 and the outlet chamber 30.

A second housing 66 defines a bore 68 therewithin having a first inletport 70 which is communicated to the inlet chamber 28 by an inlet port72 on the housing 12. Housing 66 is further provided with a second inletport 74 which is communicated to an outlet port 76 which communicates tothe outlet chamber 30. A differential pressure responsive plunger 78 isslidably mounted in the bore 68 and includes a projection 80 which isadapted to sealingly engage a valve seat 82 within the housing 66 toprevent fluid communication between the inlet port 74 and an outlet port83 which is communicated to the rear wheel brakes 84 of the vehicle.However, a spring 86 yieldably urges the projection 80 away from thevalve seat 82 to normally permit substantially uninhibited fluidcommunication between the inlet port 74 and the outlet port 83. However,when the pressure differential between the second inlet port 74 and thefirst inlet port 70 is of a sufficient magnitude to overcome the forceof the spring 86, the projection 80 is urged into sealing engagementwith the valve seat 82 to prevent further fluid communication to therear wheel brakes 84.

MODE OF OPERATION OF THE PREFERRED EMBODIMENT The various components ofthe control means are illustrated in FIG. 1 in the positions which theyassume when the brakes of the vehicle are released. When the brakes ofthe vehicle are applied, fluid pressure. is communicated into the inletchamber 28 and immediately flows through the passage 64 to the outletchamber 30, through the outlet port 76, into the bore 68, and out of theoutlet port 83 to the rear wheel brakes of the vehicle. This mode ofoperation is represented by lines A-B in FIG. 3. Although the fluidpressure levels in the chambers 30 and 28 are identical, the fluidpressure level in the chamber 30 acts across a much larger area thandoes the fluid pressure level in the chamber 28. Therefore, a force actsupon the valve members 26 and 24 urging the latter to the left, viewingFIG. 1. Since the spring 40 is sufficiently strong to maintain detentson the arms 36 and 38 and in engagement with one another, movement ofthe valve member 24 also moves the second valve member 26. However, whenthe vehicle is declerated at a predetermined rate, the mass 48accelerates to the left viewing FIG. 1, driving the tapered edges 56 ofthe resilient arms carried by the washer 52 into engagement withcorresponding serrations 58 on the guide member 46 to lock the valvemember 26 against further relative movement with the housing 12. Whenthis occurs, the valve member 24 moves relative to the valve member 26until the poppet 61 engages the passage 64 to terminate fluidcommunication therethrough. As the pressure level in the inlet chamber28 is increased, the valve reopens; however, only a portion of theincrease is transmitted to the outlet chamber 30 because of thedifferential areas between the ends 32 and 34 of the first valve means24. Therefore, a proportioning action of the valve maintains a lowerfluid pressure level in the chamber 30 than the fluid pressure levelmaintained in the chamber 28, as illustrated graphically by lines B-C inFIG. 3.

It will be apparent to those skilled in the art, that as the fluidpressure level in the inlet chamber 28 is increased, the pressurediflerential across the valve between the inlet port 16 and the outletport 76 will be increased. Since the pressure level in the inlet chamber28 is transmitted to one side of the plunger 78 within the housing 66and the pressure level in the outlet chamber 30 is transmitted to theother side of the plunger 78 in the housing 66, an increasing pressuredifferential will exist across the plunger 78 as the fluid pressurelevel in the inlet and outlet chambers is increased. Ultimately, thispressure differential will be sufficient to urge the projection intosealing engagement with the valve seat 82 to prevent further fluidcommunication to the brakes 84. This state is represented by line C-DinFIG. 3. It will be noted thatthe actual operation of the valve, asillustrated by the solid line in FIG. 3, is quite close to thetheoretically perfect brake pressure relationship between the'front andrear brakes as illustrated by the dashed lines in FIG. 3. Of course,when the vehicle is heavily loaded, a much higher fluid pressure levelis required to obtain a given vehicle deceleration. Therefore, a higher.fluid pressure level is attained, in the chamber 28 before'the mass 48is accelerated a sufficient amount to lock the housing 44 against theguide member 46 to initiate a proportioning action of the valve members24 and 26. This higher fluid pressure level is represented by point B inFIG. 3, and the proportioning action of the valve members 24 and 26 atthis higher pressure level is represented by line B C' in FIG. 3.However, it will also be noted that since the force exerted by thespring 86 is constant, the same pressure differential between thechambers 30 and 28 will result in termination of fluid communication tothe brakes 84 when the vehicle is heavily loaded as is required when thevehicle is lightly loaded. The pressure relationship between front andrear brakes after engagement of plunger 80 with seat 82 in a heavilyloaded vehicle is represented by line C D in FIG. 3. Details of theconstruction and operation of the valve 10, especially its mode ofoperation when the brakes of the vehicle are released, are morecompletely described in copending U. S. Pat. application Ser. No.171,929, filed Aug. 16, 1971, owned by the assignee of the presentinvention and incorporated herein by reference.

DETAILED DESCRIPTION OF THE ALTERNATE EMBODIMENT Referring now to FIG.2, a control valve includes a housing 112 defining a first bore 114therewithin which slidably receives a stepped piston 116. Piston 116defines a compartment 118 therewithin in which a poppet member generallyindicated by the numeral 1 19 reciprieates. Poppet member 119 includes aresilient portion 120 which is adapted to cooperate with a valve seatingarea 122 in the compartment 118 to control fluid communication through apassage 124 communicating the compartment 118 to an outlet chamber 126.The resilient portion 120 is mounted on a fluted stem 128 which extendsthrough the passage 124 to engage the end 130 of the housing 112. Theend of the fluted stem 128 is yieldably maintained in engagement withthe end 130 of the housing 112 by a spring 132. Additional spring means134 yieldably urge the end 136 of the piston 116 into engagement withthe end 130 of the bore 114. It will be noted that the fluted stem 128is sufficiently long so that as long as the end 136 of the piston 116 ismaintained in engagement with the end 130 of the bore 114, substantiallyuninhibited fluid communication is permitted between the compartment 118and the outlet chamber 126 through the passage 124 around the flutes ofthe fluted stem 128. It will also be noted that the fluid pressure levelin the compartment 1 18 is the same as the fluid pressure levelcommunicated into the inlet chamber 138 through the inlet port 140.Therefore, the effective area of the piston 116 against which inletfluid pressure acts is equal to the area identified by (A A while thefluid pressure level in the outlet chamber 126 acts across asubstantially larger area identified by the letter A disregarding thepoppet area. One end of the piston 116 extends out of one end of thebore 114 and is engaged by a torsion bar 142. The torsion bar 142reflects the relative position between the body and the rear axle of thevehicle on which the control valve 110 is mounted, and

therefore exerts an additional force on the piston 116 supplementing theforce of the springs 134 as the load carried by the rear axle of thevehicle is increased.

A differential pressure responsive plunger 144 is slidably mounted in asecond bore 146 defined within the housing 112. The plunger 144 carriesa projection 148 which is adapted to engage a valve seat 150 to preventfluid communication out of the outlet port 152 which is communicated tothe rear wheelbrakes of the vehicle. A spring 153 yieldably urges oneend of the plunger into engagement with a flange 154 provided on theleft MODE OF OPERATION OF THE ALTERNATE EMBODIMENT Variouscomponents ofthe valve 110 are illustrated in FIG. 2 in the positions which theyassume when the brakes on the vehicle are released. When a brakeapplication is effected, fluid is communicated into the compartment 118and is communicated through the passage 124 into the outlet chamber 126and then through the bore 146 to the outlet port 152. However, althoughequal fluid pressures act across areas (A -A and A on the valve piston116, a force differential exists across the piston since area A, islarger than the area (A2""A1)- When this force differential issufficient to overcome the force exerted on the piston 116 by thesprings 134 and by the torsion bar 142, the piston 116 moves to the leftviewing FIG. 2 until the resilient poppet 120 seals against the valveseating area 122 to terminate fluid communication between the inletchamber 138 and the outlet chamber 126. This state is represented bypoint B in FIG. 3. As the pressure level in inlet chamber is furtherincreased, the 'valve reopens; however, only a portion of the increasein fluid pressure is transmitted to the outlet chamber 126 which is inproportion to the area ratio between the areas (A2"A[) and A,. Thisstate is represented by lines B-C .in FIG. 3. When the fluid pressurelevel in the inlet and outlet chambers attain a value equivalent to thatrepresented by point C in FIG. 3, the pressure differential actingacross the plunger 144 is sufficient to overcome the force of the spring153, and therefore, the projection 148 is driven into sealing engagementwith the valve seat 150 to terminate further fluid communication throughthe outlet 152, as represented by line C-D in FIG. 3. If the loadcarried by the rear axle of the vehicle is increased, the torsion bar142 exerts an additional force on the piston 116 urging the "latter tothe right viewing FIG. 2, and therefore the required ,pressuredifferential between the inlet and outlet chambers 138 and 126 isincreased before proportioning action begins. Increased fluid pressurelevel at which proportioning action begins is illustrated by point B inFIG. 3, and the proportioning action of the valve at this higher vehicleload is represented by point B C in FIG. 3. However, since the 'forceexerted by the spring 153 is constant, the pressure differential betweenthe chambers 138 and 126 required to urge the projection 148 againstvalve seat 150 is the same regardles of the vehicle load. The state atwhich fluid communication through the outlet port 152 is prevented in aheavily loaded vehicle is represented by. line C D in FIG. 3. Again, itwill be noted by those skilled in the art that the ideal pressuredistributions between the front and rear brakes for both lightly andheavily loaded vehicles, as represented by the dashed lines in FIG. 3,are approximated quite closely by the operation of the valve means 110,as represented by the solid lines in FIG. 3. When the brakes of thevehicle are released, the fluid pressure level in inlet chamber 138 andin the compartment 118 is reduced, causing the higher fluid pressure inthe outlet chamber 126 to urge the valve piston 116 to the left viewingFIG. 2, thereby expanding the volume of the chamber 126 and reducing thefluid pressure level therein. When the fluid pressure level in the inletchamber 138 is reduced an amount sufficient that the fluid pressurelevel in the outlet chamber 126 is sufficiently higher than the fluidpressure level in the inlet chamber 138, the poppet 119 is urged to theleft viewing FIG. 2 relative to the piston 116 against the force of thespring 132. When this occurs, fluid communication is then againpermitted directly between the outlet chamber 126 and the inlet chamber138, until the brakes of the vehicle are fully released. At this time,the springs 134 and the force exerted on piston 116 by the torsion bar142 returns the piston 116 so that the end 136 of the latter is againurged into engagement with the end of the bore 114 as illustrated inFIG. 2.

'- I claim:

1. In a vehicle having front and rear wheel brakes: first and secondhydraulic systems communicated to said front and rear wheel brakesrespectively; fluid motor means for simultaneously generating brakingpressure in said first and second hydraulic systems; valve means in oneof said hydraulic systems permitting unrestricted fluid communicationthrough said one system until a first pressure level is attained in saidsystem whereupon said valve means restricts communication through saidone system to establish a lower pressure level at the correspondingwheel brakes than the fluid pressure level generated by said fluid motormeans, said valve means including pressure differential responsive meansterminating fluid communication to said corresponding wheel brakes whenthe pressure differential across said valve means attains apredetermined level; and sensing means responsive to the rate ofdeceleration of the vehicle to vary said first pressure level as therate of deceleration wanes. 2. In a vehicle having from and rear wheelbrakes: first and .second hydraulic systems communicated to said frontand rear wheel brakes respectively; fluid motor means for simultaneouslygenerating braking pressure in said first and second hydraulic systems;

valve means in one of said hydraulic systems permitting unrestrictedfluid communication through said one system until a first pressure levelis attained in said system whereupon said valve means restrictscommunication through said one system to establish a lower pressurelevel at the corresponding wheel brakes than the fluid pressure levelgenerated by said fluid motor means, said valve means including pressurediflerential responsive means terminating fluid communication to saidcorresponding wheel brakes when the pressure differential across saidvalve means attains a predetermined level; and

means responsive to the weight carried by the axle of the vehiclemounting said corresponding set of wheel brakes to increase said firstpressure level as the load carried by the vehicle increases.

3. The invention of claim 2:

said one hydraulic system being communicated to the rear wheel brakes ofthe vehicle.

4. The invention of claim 2:

said valve means including housing means defining a first bore therein,valve members within said first bore dividing the latter into an inletchamber communicated to said fluid motor means and an outlet chambercommunicated to said corresponding wheel brakes, said pressuredifferential responsive means responding to the pressure differentialbetween said inlet and outlet chambers.

5. The invention of claim 4:

said pressure differential responsive means including a plunger, valvemechanism responsive to movement of said plunger to terminate fluidcommunication between said outlet chamber and said corresponding wheelbrakes, and means communicating the pressure level in said inlet chamberto one side of said plunger and the pressure level in said outletchamber to the other side of said plunger.

6. The invention of claim 4:

said pressure differential responsive means including a second boredefined within said housing means, a plunger slidably mounted withinsaid second bore and defining a pair of compartments between oppositeends of the plunger and corresponding ends of the second bore;

one of said compartments being communicated to said inlet chamber, theother compartment being communicated to said outlet chamber and to saidcorresponding wheel brakes; and

second valve means in said second bore for terminating fluidcommunication between said other compartment and said correspondingwheel brakes upon sliding of said plunger when the pressure differentialbetween said compartments attains a predetermined level.

7. The invention of claim 6; and

resilient means yieldably urging said plunger toward one end of saidbore, whereby the pressure differential between said compartments mustexceed the force exerted by said resilient means before said plungerslides.

8. The invention of claim 6:

said second valve means including a first valve member carried by saidplunger and a second valve member mounted in said second compartment,said plunger sliding in said bore when a pressure differential of apredetermined magnitude is established across said plunger to urge saidfirst valve member into sealing engagement with said second valve memberto thereby prevent fluid communication to said corresponding wheelbrakes.

1. In a vehicle having front and rear wheel brakes: first and secondhydraulic systems communicated to said front and rear wheel brakesrespectively; fluid motor means for simultaneously generating brakingpressure in said first and second hydraulic systems; valve means in oneof said hydraulic systems permitting unrestricted fluid communicationthrough said one system until a first pressure level is attained in saidsystem whereupon said valve means restricts communication through saidone system to establish a lower pressure level at the correspondingwheel brakes than the fluid pressure level generated by said fluid motormeans, said valve means including pressure differential responsive meansterminating fluid communication to said corresponding wheel brakes whenthe pressure differential across said valve means attains apredetermined level; and sensing means responsive to the rate ofdeceleration of the vehicle to vary said first pressure level as therate of deceleration wanes.
 2. In a vehicle having front and rear wheelbrakes: first and second hydraulic systems communicated to said frontand rear wheel brakes respectively; fluid motor means for simultaneouslygenerating braking pressure in said first and second hydraulic systems;valve means in one of said hydraulic systems permitting unrestrictedfluid communication through said one system until a first pressure levelis attained in said system whereupon said valve means restrictscommunication through said one system to establish a lower pressurelevel at the corresponding wheel brakes than the fluid pressure levelgenerated by said fluid motor means, said valve means including pressuredifferential responsive means terminating fluid communication to saidcorresponding wheel brakes when the pressure differential across saidvalve means attains a predetermined level; and means responsive to theweight carried by the axle of the vehicle mounting said correspondingset of wheel brakes to increase said first pressure level as the loadcarried by the vehicle increases.
 3. The invention of claim 2: said onehydraulic system being communicated to the rear wheel brakes of thevehicle.
 4. The invention of claim 2: said valve means including housingmeans defining a first bore therein, valve members within said firstbore dividing the latter into an inlet chamber communicated to saidfluid motor means and an outlet chamber communicated to saidcorresponding wheel brakes, said pressure differential responsive meansresponding to the pressure differential between said inlet and outletchambers.
 5. The invention of claim 4: said pressure differentialresponsive means including a plunger, valve mechanism responsive tomovement of said plunger to terminaTe fluid communication between saidoutlet chamber and said corresponding wheel brakes, and meanscommunicating the pressure level in said inlet chamber to one side ofsaid plunger and the pressure level in said outlet chamber to the otherside of said plunger.
 6. The invention of claim 4: said pressuredifferential responsive means including a second bore defined withinsaid housing means, a plunger slidably mounted within said second boreand defining a pair of compartments between opposite ends of the plungerand corresponding ends of the second bore; one of said compartmentsbeing communicated to said inlet chamber, the other compartment beingcommunicated to said outlet chamber and to said corresponding wheelbrakes; and second valve means in said second bore for terminating fluidcommunication between said other compartment and said correspondingwheel brakes upon sliding of said plunger when the pressure differentialbetween said compartments attains a predetermined level.
 7. Theinvention of claim 6; and resilient means yieldably urging said plungertoward one end of said bore, whereby the pressure differential betweensaid compartments must exceed the force exerted by said resilient meansbefore said plunger slides.
 8. The invention of claim 6: said secondvalve means including a first valve member carried by said plunger and asecond valve member mounted in said second compartment, said plungersliding in said bore when a pressure differential of a predeterminedmagnitude is established across said plunger to urge said first valvemember into sealing engagement with said second valve member to therebyprevent fluid communication to said corresponding wheel brakes.